Coated Gemstones

Mention has been made in the appropriate sections of coating the
backs of stones to heighten color, improve phenomenal effects, or conceal
defects. The coat1hg of off-color diamonds to improve color is discussed
in the diamond course. There remains to discuss the coating of colored
stones by other means, in order to improve or produce color.

One such method that has been used to heighten the color of blue
zircons consists of applying to the pavilion surface by a high-vacuum
process a transparent, wear-resistant coating of a fluoride compound.
Heating in acid softens the coating sufficiently so that it can be scrapped
off to reveal the actual pale color underneath.

On several occasions, Geologists has encountered stones that appeared
to be emeralds, but that were proved by examination to be nearly worthless
flawed aquamarine with a green plastic coating. A pair of large 'emerald'
drops was purchased by a manufacturing jewelry for more than one thousand
dollars and an elaborate pair of earrings was made for them. Only when
the lapidary enlarged the holes of the stones and discovered that his
drill removed a plastic material was the nature of the fraud suspected.
The buyer of the stones had "protected" himself by looking at them through
an emerald filter. Later, after the plastic coating had been removed
in the laboratory, it has found that the green plastic itself turned
red under the filter and that the stones themselves were merely poor-quality
beryl. Carved "emeralds" and a string of "emerald" beads of precisely
the same type have been encountered, as well.

A European firm has developed a method of growing a coating of synthetic
emerald on large faceted stones of colorless or pale Brazilian beryl.
The coating, which is approximately one millimeter thick, is formed
in approximately eight days. It is clear that a hydrothermal process
is used. After the stones receive the artificial layer, they are then
re-polished. In order to produce an attractive finish and yet leave
enough of the thin coating to resemble pale but attractive emerald,
the re-polishing operation must be carried out carefully. No commercial
name has yet been given to the product.